It is known that at low frequencies the four-terminal pairs impedance measuring method is the measuring method which is not affected by the measurement cables that connect the device under test (hereinafter DUT) with the main body of the measurement equipment. For example, accurate measurements are possible even with long measurement cables in designs such as those that provide a multiplexer between the DUT and the main body of the measurement equipment, so that they can switch between the DUT and the main body of the measurement equipment.
However, as the measurement frequency increases, the phase shift within the measurement cables lowers the accuracy of the measurement, and the feedback loop that is a part of the measurement equipment becomes less stable. In order to stabilize the feedback loop, strict constraints are placed on the feedback loop and on the measurement cables. Therefore, there is no freedom in the selection of the measurement cables.
An explanation will be provided herein of the problems associated with a typical impedance measuring device using four-terminal pairs, as illustrated in FIG. 2. Specifically, the discussion will involve the problems associated with the feedback loop and with increasing the frequency or using measurement cables of arbitrary length. Note that while not shown in FIG. 2 and in subsequent figures, the impedance measuring equipment further includes a portion that calculates measured values, and a portion that controls the functions of the measurement equipment.
In the operation of the impedance measuring equipment as shown FIG. 2, the measurement current from a measurement signal generator 13 is supplied through a resistor 12, a measurement cable 11, and a High Current (Hc) measurement terminal 10, to a DUT 60. The measurement current flows through another terminal 62 of DUT 60, a Low Current (Lc) measurement terminal 40, a measurement cable 41, a range resistor 43 used as a current detector, and is pulled into a null amplifier stage 35. The current value is then determined by measuring the voltage across resistor 43. In other words, resistor 43 and voltmeter 44 form a current measuring stage 42.
At the same time, the electric potential of terminal 62 of DUT 60 goes from a Low Potential (Lp) measurement terminal 30, through a measurement cable 31, and is applied to null amplifier stage 35. The output of null amplifier stage 35 pulls in the current from an ammeter 42, so that the electric potential of terminal 62 of DUT 60 is the same as the ground potential of null amplifier stage 35. That is to say, the feedback loop comprising measurement cable 31, null amplifier stage 35, current measuring stage 42, measurement cable 41, and terminal 62 of DUT 60 performs negative feedback control and, thus, maintains the electric potential of terminal 62 of DUT 60 at a zero potential. This type of feedback loop is referred to as a null loop.
The electric potential of terminal 61 of DUT 60 passes through a High Potential (Hp) measurement terminal 20 and a measurement cable 21 and is measured by a voltmeter 22. As explained above, the electric potential of terminal 62 is maintained at the ground potential, so that voltmeter 22 can measure the voltage across DUT 60 (i.e., across Lp and Hp terminals). The desired measured value of the impedance is then computed as the ratio of the measured value of voltmeter 22 and the measured value of current measuring stage 42.
However, if the phase margin of the open loop transfer function of the null loop is not sufficient, the null loop becomes unstable and oscillates which makes it impossible to utilize the measurement circuit using four-terminal pairs. One approach to remedying the stability problem is to employ phase compensation within the null loop.
In the past, the following method has been utilized for phase compensation of the null loop. As shown in FIG. 2, a null amplifier stage 35 is utilized to provide the necessary phase compensation and includes an input amplifier 32, a narrow band high gain amplifier 33, and an output amplifier 34 in series.
FIG. 3 illustrates the design of narrow band high gain amplifier 33, which operates as follows. A synchronizing signal from a synchronizing signal generator 79, having the same frequency as the measurement signal generator, is directly applied as a reference phase signal to a phase detector 71. The synchronizing signal is also applied as a reference phase signal to a phase detector 72, after being phase shifted 90 degrees by a phase shifter 80. Phase detectors 71 and 72 are thus orthogonal synchronized phase detectors.
The alternating current signal that is applied to the input terminal 70 is split into two orthogonal components by phase detectors 71 and 72 and detected synchronously, resulting in direct current signals.
The synchronizing signal from synchronizing signal generator 79 goes through a variable phase shifter 81 and is applied as a carrier wave to a modulator 75, and also to a modulator 76 after being phase shifted 90 degrees at a phase shifter 82. Therefore, modulators 75 and 76 are orthogonal modulators.
The direct current signals are integrated at integrators 73 and 74, respectively, and then inputted to modulators 75 and 76 which transform the direct current signals into alternating currents with a phase shift difference of 90 degrees. The alternating currents are then combined at an adder 77 to regenerate an alternating current signal which is outputted via output 78.
In this way, narrow band high gain amplifier 33 performs orthogonal synchronous phase detection of an alternating current signal to transform it into a direct current signal, integrates the direct current signal, and performs orthogonal modulation to return it to an alternating current signal, thus achieving a high gain in a narrow band. FIG. 4 illustrates an example of the gain characteristic and the phase characteristics when the phase shift amount of the phase shifter are zero.
Note that it is possible to create a narrow band high gain amplifier with an arbitrary phase difference by using variable phase shifter 81 to shift the phase between the orthogonal phase detectors and the orthogonal modulators. FIG. 5 shows an example of the phase characteristics as the phase difference between the orthogonal phase detectors and the orthogonal modulators is varied at 0 degrees, +90 degrees, and -90 degrees.
An important condition for maintaining stability in the null loop is that a phase of 0 degrees does not exist within the gain bandwidth of the null loop containing narrow band high gain amplifier 33. Therefore, if the conditions are strictly determined at the outset, such as the electrical length of the measurement cables, it is possible to adjust the phase shift amount of narrow band high gain amplifier 33 and to build stability into the null loop prior to manufacture and shipping. In the alternative, if the impedance measuring equipment has the ability to determine the necessary phase shift amount (phase compensation amount) during use, it is possible to provide a very flexible solution with regard to the cable length.
An example of the latter approach is provided in Japanese Laid-Open Publication Number 03-61863 by the present applicant(s) entitled "Adaptive Type Half-Bridges And Impedance Meters." The device determines and specifies a phase shift amount in order to stabilize the feedback loop, based on the phase characteristics of the measured feedback loop. In particular, the device separates the measurement signal generator from the measurement circuit, cuts off the null loop, utilizes an additional circuit to measure the phase shift amount of the null loop exclusive of narrow band high gain amplifier 33, and then sets a variable phase shifter 81 so that the total phase shift amount of the null loop is 180 degrees.
Referring to FIG. 6, which illustrates a corresponding conceptual diagram of the above-mentioned device, the operation of the above-mentioned device will be described. Initially, switch 14 is flipped to the ground side, which disconnects signal generator 13 from the measurement circuit. Switch 36 is then flipped to the side of an input signal generator 38. As a result, the null loop is cut off and the input signal E.phi. of input signal generator 38 is applied to output amplifier 34. The voltage V.phi. that appears on the output side of an input amplifier 32 is measured by a vector voltmeter 37. The phase difference between V.phi. and the input signal E.phi. is the phase shift amount of the null loop exclusive of narrow band high gain amplifier 33. Based on the determined phase shift amount, variable phase shifter 81 (FIG. 3) is adjusted so that the total phase shift amount of the null loop is 180 degrees.
The actual phase measurement can be performed by using a phase measurement circuit incorporated into a narrow band high gain amplifier 33, as shown in FIG. 7.
Referring to FIG. 7, an operational amplifier 86, resistor 85 and capacitor 84 constitute an integrator. A resistor 83 and a switch 91 are coupled in series between an input and output of operational amplifier 86, such that when switch 91 is closed, the integrator switches over to the amplifier. The same also applies for an operational amplifier 90, resistor 89, capacitor 88, resistor 87, and switch 91.
When the phase is measured, the phase shift amount of variable phase shifter 81 is set to zero or some known value. Switch 95 is used to ground the input of modulator 76 or to connect the input of modulator 75 to a direct current power supply 94. The direct current voltage of direct current power supply 94 is converted to an alternating current signal that goes around the null loop, passes through an input 70 and a buffer 96, and is applied to phase detectors 71 and 72. If switch 91 is set in the closed position to switch from the integrator to the voltage amplifier, and switch 92 causes the direct current voltage that is split into orthogonal components by phase detectors 71 and 72 to be measured by voltmeter 93, the phase can then be determined as an argument in the complex plane.
In the entire process of the impedance measurement, the stage that performs the phase measurement in Japanese Laid-Open Publication Number 03-61863 can take either of the following two forms.
(1) When the DUT is connected and the measurement of the impedance is initiated, the first half of each measurement cycle is allocated to determining the phase compensation amount of the null loop. PA1 (2) The phase compensation amount of the null loop is determined at system construction time (such as when the cables are extended) with the DUT in the open condition, and the result is stored in the equipment's memory. The stored value of the compensation amount can be retrieved and utilized to measure an impedance.
It is important to understand that the total phase shift of the null loop is not the arithmetic sum of the respective effects of the cable lengths and the impedance value of the DUT. In other words, except under special conditions, the phase shift cannot be apportioned to a phase shift function based on the impedance value of the DUT plus the function for the cable lengths.
The above-mentioned forms (1) and (2) have complementary advantages and disadvantages. Form (1) makes it possible to obtain extremely flexible stability with respect to the value of the DUTs and the cable lengths. However, additional time is required to measure the phase, which reduces the overall measurement speed.
Form (2) has no measurement speed overhead. However, it does not guarantee the stability of the null loop with respect to arbitrary values of the DUTs because the null loop transfer function changes according to the value of the DUTs. Therefore, it is necessary to utilize the phase difference obtained from some DUTs in order to determine an optimal phase compensation amount that will guarantee the phase margin across the entire range of DUTs. The only way to achieve flexibility, however, is to predict the phase shift based on the internal design of the impedance measurement equipment and to have a specially measured cable length.
In general, impedance measuring equipment that utilizes a four-terminal pair method includes a feedback loop which receives a current from a DUT in order to measure the current, while keeping the electric potential of the measured terminals at the ground potential. When making remote measurements at high frequencies, the feedback loop may become unstable and measurement may not be possible. One method to remedying the stability problem is disclosed in Japanese Laid-Open publication Number 03-61863 (as described above). However, the method either results in restrictions on the measurement cables or increases the measurement time.
Accordingly, it is an object of the present invention to provide a method for accurately determining the stabilizing compensation amount with the greatest margin across the entire range of values of the DUT, without sacrificing measurement time.